Thermally switched optical downconverting filter

ABSTRACT

A thermally switched optical downconverting (TSOD) filter is a self-regulating device including a downconverter that converts incoming light at a variety of wavelengths into longer-wavelength radiation and then directs it using one or more bandblock filters in either the inward or outward direction, depending on the temperature of the device. This control over the flow of radiant energy occurs independently of the thermal conductivity or insulating properties of the device and may or may not preserve the image and color properties of incoming visible light. The TSOD filter has energy-efficiency implications, as it can be used to regulate the internal temperature and illumination of buildings, vehicles, and other structures without the need for an external power supply or operator signals. The TSOD filter also has aesthetic implications, since the device has unique optical properties that are not found in traditional windows, skylights, stained glass, light fixtures, glass blocks, bricks, or walls. The TSOD filter has particular, but not exclusive, application as a building material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority pursuant to 35 U.S.C.§119(e) of U.S. provisional patent application No. 60/897,184 filed 24Jan. 2007 and U.S. provisional patent application No. 60/931,068 filed21 May 2007, each of which is hereby incorporated herein by reference inits entirety.

BACKGROUND

1. Technical Field

The subject matter described herein relates to solid-state and “nearlysolid state” devices for controlling the flow of light and radiant heatthrough downconversion and selective reflection. The technology hasparticular, but not exclusive, application in passive or activetemperature-regulating films, materials and devices, especially asconstruction materials.

2. Description of the Related Art

Photodarkening materials have been used for decades, for example insunglass lenses, to selectively attenuate incoming light when stimulatedby ultraviolet (UV) radiation. When incorporated into windows, suchmaterials can be used to regulate the internal temperature of astructure by darkening to attenuate bright sunlight and by becomingtransparent again to allow artificial light or diffuse daylight to passthrough unimpeded. Such systems are passive and self-regulating,requiring no external signal other than ambient UV light in order tooperate. However, because they are controlled by ultraviolet rather thanby temperature, such systems are of limited utility intemperature-regulating applications.

Electrodarkening and photodarkening materials attenuate incoming lightprimarily through absorption rather than reflection, meaning they willheat up when exposed to bright light. This creates a conductive heatflux which offsets the reductions in radiative transmission and thusplaces significant limits on their ability to regulate temperature.

The process of absorbing one wavelength of light and emitting another,longer wavelength of light is known as downconversion. This processoccurs in a number of naturally occurring fluorescent and phosphorescentmaterials, including phosphorus. Blackbody radiation from anenergy-absorbing material is a form of downconversion as well.Downconversion also occurs in semiconductor materials, which absorbenergy over a wide band of wavelengths and emit energy in a muchnarrower band, centered on the bandgap energy of the material, through aprocess known as photoluminescence. A downconverter can easily befashioned from a piece of bulk semiconductor.

The fabrication of very small structures to exploit the quantummechanical behavior of charge carriers, e.g., electrons or electron“holes” is well established. Quantum confinement of a carrier can beaccomplished by a structure having one or more dimensions less than thequantum mechanical wavelength of the carrier. Confinement in a singledimension produces a “quantum well,” and confinement in two dimensionsproduces a “quantum wire.”

A quantum dot is a structure capable of confining carriers in all threedimensions. Quantum dots can be formed as particles, with dimensions inall three directions of less than the de Broglie wavelength of a chargecarrier. Quantum confinement effects may also be observed in particlesof dimensions less than the electron-hole Bohr diameter, the carrierinelastic mean free path, and the ionization diameter, i.e., thediameter at which the quantum confinement energy of the carrier is equalto its thermal-kinetic energy. It is postulated that the strongestconfinement may be observed when all of these criteria are metsimultaneously. Such particles may be composed of semiconductormaterials (for example, Si, GaAs, AlGaAs, InGaAs, InAlAs, InAs, andother materials), or of metals, and may or may not possess an insulativecoating. Such particles are referred to in this document as “quantum dotparticles.”

Quantum dots can have a greatly modified electronic structure from thecorresponding bulk material, and therefore different properties. Becauseof their unique properties, quantum dots are used in a variety ofelectronic, optical, and electro-optical devices. Quantum dots arecurrently used as near-monochromatic fluorescent light sources, laserlight sources, light detectors including infrared (IR) detectors, andhighly miniaturized transistors, including single-electron transistors.

The embedding of metal and semiconductor nanoparticles inside bulkmaterials (e.g., cadmium sulfide particles as a colorant in ornamentalcrystal) has been practiced for centuries. However, an understanding ofthe physics of these materials has only been achieved comparativelyrecently. These nanoparticles are quantum dots with characteristicsdetermined by their size and composition. These nanoparticles serve asdopants for the material in which they are embedded to alter selectedoptical or electrical properties. The “artificial atoms” represented bythese quantum dots have properties which differ in useful ways fromthose of natural atoms. However, it must be noted that the dopingcharacteristics of the quantum dots are fixed at the time of manufactureand cannot be adjusted thereafter.

Leatherdale et al., “Photoconductivity in CdSe Quantum Dot Solids,”Physics Review B (15 Jul. 2000), describe, in detail, the fabrication of“two- and three-dimensional . . . artificial solids with potentiallytunable optical and electrical properties.” These solids are composed ofcolloidal semiconductor nanocrystals deposited on a semiconductorsubstrate. The result is an ordered, glassy film composed of quantum dotparticles, which can be optically stimulated by external light sourcesor electrically stimulated by electrodes attached to the substrate toalter optical and electrical properties.

U.S. Pat. No. 5,881,200 to Burt discloses an optical fiber (1)containing a central opening (2) filled with a colloidal solution (3) ofquantum dots (4) in a support medium. The purpose of the quantum dots isto produce light when optically stimulated, for example, to produceoptical amplification or laser radiation. The quantum dots take theplace of erbium atoms, which can produce optical amplifiers when used asdopants in an optical fiber. The characteristics of the quantum dots canbe influenced by the selection of size and composition at the time ofmanufacture. Although this device has an input or source path and anoutput or drain path, it does not have means of external control, and sois not a “switch” in any meaningful sense. As such, it does not preventor regulate the flow of light energy through the fiber.

Goldhaber-Gordon et al., “Overview of Nanoelectronic Devices,”Proceedings of the IBEE, Vol. 85, No. 4, (April 1997), describe what maybe the smallest possible single-electron transistor. This consists of a“wire” made of conductive C₆ benzene molecules with a “resonanttunneling device,” or “RTD,” inline that consists of a benzene moleculesurrounded by CH₂ molecules, which serve as insulators. The device isdescribed, perhaps incorrectly, as a quantum well (rather than a quantumdot) and is intended as a switching device transistor rather than aconfinement mechanism for charge carriers. However, in principle thedevice should be capable of containing a small number of excesselectrons and thus serving as a quantum confinement device. Thus, theauthors remark that the device may be “much more like a quantum dot thana solid state RTD.” (See p. 19.)

U.S. Pat. No. 6,512,242 to Fan et al. describes a device for producingquantum effects comprising a quantum wire (504), energy carried alongthe quantum wire under voltage control, and quantum dots (502, 503) nearthe quantum wire that hold energy. The quantum wire transports electronsinto and out of a quantum dot or plurality of quantum dots through“resonant tunneling.” As described by Fan et al., the quantum dots serveas “resonant coupling elements” that transport electrons along thequantum wire acting as an electronic waveguide or between differentports on the same waveguide. In other words, the quantum dots serve as akind of conduit.

U.S. patent application publication No. 2002/0079485A1 by Stinz et al.discloses a “quantum dash” device that can be thought of as anon-spherical, non-radially-symmetric quantum dot particle withelongated axes, or as a short, disconnected segment of quantum wire. Inthis sense, quantum dashes are merely a special class of quantum dotparticles. As described by Stinz et al., pluralities of the quantum dashdevices are embedded at particular locations inside a solid material toenhance the excitation of laser energy within the material. Theresulting structure is a “tunable laser” with an output frequency thatcan be adjusted over a narrow range. This tuning is accomplished through“wavelength selective feedback” using an external optical grating tolimit the input light frequencies that can reach the dashes inside thematerial. The publication notes that “an ensemble of uniformly sizedquantum dashes that functioned as ideal quantum dots would have anatomic-like density of states and optical gain.” Stinz et al. relies onthe exact geometry and composition of the semiconductor material toproduce quantum dashes of a particular size and shape. Therefore,selection of the available quantum states is achieved exclusively at thetime of manufacture, “with a variety of length-to-width-to-heightratios, for example, by adjusting the InAs monolayer coverage, growthrate, and temperature.” The energy affects all the quantum dashesequally, along with the surrounding material in which they are embedded,and if the surrounding material is opaque, then photon energy cannotreach the quantum dashes at all. Again, this device is not an opticalswitch.

U.S. Patent Application Publication No. 2002/0114367A1 by Stinz et al.discloses “an idealized quantum dot layer that includes a multiplicityof quantum dots embedded in a quantum well layer sandwiched betweenbarrier layers.” Similarly, U.S. Pat. No. 6,294,794 B1 to Yoshimura etal. discloses “a plurality of quantum dots in an active layer such thatthe quantum dots have a composition or doping modified asymmetric in adirection perpendicular to the active layer.” These quantum dotparticles are simply embedded in an optical crystal. A similar quantumdot layer structure is disclosed in U.S. Pat. No. 6,281,519 B1 toSugiyama et al.

McCarthy, et al., in U.S. Pat. No. 6,978,070, discloses in detail aplurality of bank-addressable quantum dot devices which can be used asprogrammable dopants to alter the bulk electrical, optical, thermal,magnetic, chemical, and mechanical properties of a substrate (whethercylindrical, flat, or some other shape) in a controlled and repeatableway. This control could take place not only at the time of fabricationof the material, but also in real time, i.e., at the time of use, inresponse to changing needs and conditions.

McCarthy et al, in U.S. Patent Application Publication No. 2006/0011904,discloses a layered composite film incorporating quantum dots asprogrammable dopants. A means is described in detail for controllinglarge numbers of quantum dots in order to affect the bulk properties ofa substrate near its surface. The device may incorporate switches inorder to turn power on and off to control wires or control wirebranches, but these switches are not thermally controlled. The authorsalso note that the device “can . . . be used as a solid-state thermalswitch, i.e., it can be switched between thermally conductive andthermally insulating states, forming the thermal equivalent of anelectronic transistor or rheostat.” However, the configuration of such athermal switch is not specified, e.g., the input and output paths arenot drawn or described, although the source, drain, and gate of theswitches (122) in the control wires are clearly shown.

Harrison, “Quantum Wells, Wires, and Dots,” John Wiley & Sons, Ltd.(2000) notes the existence of a “two dimensional electron gas fieldeffect transistor (TEGFET) . . . a type of High Electron MobilityTransistor (HEMT) designed to exploit the high in-plane (x-y) mobilitywhich “arises when a . . . heterojunction is modulation-doped.” Thisdesign includes the one-dimensional quantum confinement of carriers(i.e., confinement along the z-axis) which can occur at a heterojunction(i.e., at the interface between two electrically dissimilar materials).However, since carriers are only free to travel in the x-y plane, andsince there is no quantum confinement in the x or y direction, theone-dimensional quantum confinement is incidental rather than exploited,and does not play a necessary role in the functioning of the device.While this device is certainly a switch, it is neither optical in naturenor thermally controlled.

Harrison also discloses an effect known as the Quantum Confined StarkEffect, wherein an electric field is applied perpendicular to a quantumwell to affect the energy level of the carriers confined within it.While this is known to have a slight effect on the absorption spectrumof the quantum well, the effect is exploited in sensors rather than inswitches. In addition, Harrison does not state or imply that the StarkEffect has ever been used to modify the behavior of a TEGFET device orany other type of switch.

There is one other type of switch that relies on quantum confinement:the single-electron transistor or SET. This consists of a source (input)path leading to a quantum dot particle or quantum dot device, and adrain (output) path exiting, with a gate electrode controlling the dot.With the passage of one electron through the gate path into the device,the switch converts from a conducting or closed state to a nonconductingor open state, or vice-versa. However, SETs are not designed to controlthe flow of thermal or optical energy and do not incorporate opticaldownconverters or bandblock filters.

Thermal switches also exist, which allow the passage of heat energy intheir ON or closed state, but prevent it in their OFF or open state.However, these switches are mechanical relays, which rely on contactbetween two conducting surfaces (typically made of metal) to enable thepassage of heat. When the two surfaces are withdrawn, heat energy isunable to conduct between them except through the air gap. If the deviceis placed in vacuum, heat conduction is prevented entirely in the openstate. Another type of thermal switch involves pumping a gas or liquidin or out of a chamber. When the chamber is full, it conducts heat. Whenempty, there is no conduction. Notably, these devices are notsolid-state, not multifunctional, not programmable, and do not rely onquantum confinement for their operation.

Optical switches also exist. Light can be blocked by optical filterswhich absorb or reflect certain frequencies of light while allowingothers to pass through. Shortpass and longpass filters may be used or anarrow range of frequencies can be blocked by a notch filter orbandblock filter. Some filters today also incorporate quantum wells,quantum wires, or quantum dot particles.

The addition of a mechanical shutter can turn an otherwise transparentmaterial13 including a filter—into an optical switch. When the shutteris open, light passes through easily. When the shutter is closed, nolight passes. If the mechanical shutter is replaced with anelectrodarkening material such as a liquid crystal, then the switch is“nearly solid state,” with no moving parts except photons, electrons,and the liquid crystal molecules themselves. Other electrodarkeningmaterials, described for example in U.S. Pat. No. 7,099,062 to Azens etal., can serve a similar function. It will be clear to a person ofordinary skill in the art that these optical filter/switch combinationsare not passive, but must be operated by external signals.

The information included in this Background section of thespecification, including any references cited herein and any descriptionor discussion thereof, is included for technical reference purposes onlyand is not to be regarded as subject matter by which the scope of theinvention is to be bound.

SUMMARY

The technology disclosed herein is directed to the separate control overthe thermal conductivity and transmissivity of a material with regard toradiant energy for the purpose of regulating the flow of heat in usefulways, without necessarily retaining the optical characteristics of theradiant energy passing through the material. In particularimplementations the technology employs a temperature-responsive opticaldownconverter, sandwiched between two notch or bandblock filters ofdifferent center wavelength or placed adjacent to a single object, toregulate the passage of light energy such that most of the incidentenergy passes through the device when it is below a thresholdtemperature, and such that most of the incident energy is reflected, orabsorbed and re-radiated, or otherwise directed away from the deviceabove a second threshold temperature, yielding a thermally switchedoptical downconverting filter (hereinafter a “TSOD filter”).

The TSOD filter exhibits three distinct behaviors. At a low temperaturerange or below a threshold temperature, light energy passes through theTSOD filter. Over an intermediate temperature range, the TSOD filterreflects or radiates away approximately half of the light energy thatstrikes it, and transmits the other half. Over a high temperature rangeor above a threshold temperature, the TSOD filter reflects or radiatesaway almost all of the incident light energy. Thus, the TSOD filter canbe used to regulate the internal temperatures of buildings and otherstructures by controlling the amount of solar radiation they absorb.

The TSOD filter is a passive, self-regulating device—a so-called smartmaterial—which requires no external signals or user inputs in order tofunction. The TSOD filter thus functions as a solid-state opticalswitch. The switch contains no moving parts, other than photons andelectrons. The TSOD filter further provides for regulation, based ontemperature, of the amount of light energy that passes through it. Thispermits control over the internal temperatures of buildings, vehicles,and other structures by controlling the absorption of solar energy orother incident light energy.

The physical instantiation of the TSOD filter may be thick or thin,strong or weak, rigid or flexible, monolithic or made up of separateparts, without altering its basic function in any significant way. Whenthe TSOD filter is configured to transmit little or no visible light, itmay serve as an aesthetic, energy-regulating replacement for opaquebuilding materials such as wood, brick, fiberglass, and drywall. Whenthe TSOD filter is configured to transmit diffuse or attenuated visiblelight, it may serve as an aesthetic, energy-regulating replacement fortranslucent building materials such as glass block, privacy glass, andtextured polymers. When the TSOD filter is configured to transmitvisible light with little diffusion or attenuation, it may serve as anaesthetic, energy-regulating replacement for transparent buildingmaterials such as glass or polymer windows. When the downconverter inthe TSOD filter is configured to emit monochromatic light in the visiblespectrum, it may serve as a bright, energy-regulating replacement forstained glass, tinted windows, window appliques and coatings, or coloredartificial light sources.

Other features, details, utilities, and advantages of the presentinvention will be apparent from the following more particular writtendescription of various embodiments of the invention as furtherillustrated in the accompanying drawings and defined in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

Please note that closely related elements have the same element numbersin all figures.

FIG. 1 is a schematic, cross-section representation of one embodiment ofa TSOD filter depicting a layer of downconverter material sandwichedbetween two bandblock filters and attached to a transparent substrate.The action of incoming light is depicted for the cold state of the TSODfilter.

FIG. 2 is a schematic, cross-section representation of the embodiment ofFIG. 1, except that the action of incoming light is depicted for the hotstate of the TSOD filter.

FIG. 3 is a diagram of light intensity vs. wavelength depicting theemission spectrum of the external light source, the fluorescencespectrum or photoluminescence spectrum of the downconverter in cold andhot states, and the reflection spectrum of the two bandblock filters forone implementation of a TSOD filter.

FIG. 4 is a schematic, cross-section representation of anotherembodiment of the TSOD filter, in which there are holes in thedownconverter to allow some white light from the external source to passthrough the TSOD filter without modification.

FIG. 5 is a schematic, cross-section representation of an additionalembodiment of a TSOD filter in which a number of optional componentshave been added in order to improve the performance and aesthetics ofthe device.

FIG. 6 is a schematic, cross-section representation of a furtherembodiment of the TSOD filter in which the white light from the externallight source is passed through a concentrating lens before striking thedownconverter.

FIG. 7 is a schematic, cross-section representation of furtherembodiment of the TSOD filter, wherein the structural substrate andbandblock reflector have been combined into a single component, athermochromic attenuator is employed near the “building exterior” sideof the device, and the downconverter has been relocated to the “buildinginterior” surface of the device.

FIG. 8 is a schematic, cross-section representation of anotherembodiment of the TSOD filter, wherein the downconverter also serves asa thermochromic attenuator.

FIG. 9 is a schematic, cross-section representation of an additionalembodiment of the TSOD filter, wherein a thermochromic attenuator islocated on the inside surface of the transparent substrate, while thebandblock filter and transparent insulator have been combined as asingle component.

FIG. 10 is a schematic, cross-section representation of a furtherembodiment of the TSOD filter with potential specific application in theform of a spandrel.

DETAILED DESCRIPTION

The disclosed technology is directed to the use of a thermochromicoptical downconverter, in conjunction with one or more bandblockfilters, to create a thermally switched optical downconverting filter(TSOD filter) that allows light energy to pass through at low ambienttemperatures and reflects or radiates it away at high ambienttemperatures. The technology is well suited for use in buildingmaterials such as spandrels, windows, and siding to passively regulatethe heating and cooling of a building or other structure. For example,in the winter when the ambient air temperature is cold, buildingmaterials incorporating a TSOD filter can pass the majority of the solarenergy from incident sunlight into the interior of the building topassively heat the building. Likewise, in the summer when the ambientair temperature is hot, building materials incorporating a TSOD filtercan block the majority of the solar energy from incident sunlight frompassing into the interior of the building and thus keep the buildingcool.

For the purposes of this document, the term “switch” includessolid-state, chemical, and mechanical devices for selectively blockingor permitting the flow of energy, and includes both digital switches(e.g., transistors and relays) and analog regulators (e.g., tubes andrheostats). Furthermore, a valve for selectively blocking or regulatingthe flow of gases or fluids can be considered analogous to a switch, sothat in principle the two terms can be used interchangeably. By thisdefinition, the TSOD filter is a solid-state optical switch, which movesfrom its “open” or transmissive state to its “closed” orreflective/radiation-blocking state based on the temperature of thedevice.

For the purposes of this document, the term “passive” refers to anobject or device which responds to environmental conditions but operatesindependently of external signals or instructions from an operator.Thus, a device may include a number of complex components—even movingparts—and still be regarded as “passive” for the purposes of thisdocument. Similarly, the possible existence of a user override mode doesnot alter, in any essential way, the passive nature of such a device. Bycontrast, an active device is one that requires user input in order toperform its normal functions. (By these definitions, light-sensitivesunglasses are a passive device, whereas a standard light bulb operatedby a wall switch is an active device.)

The TSOD filter includes a downconverter, which absorbs incoming light(e.g., sunlight) at a variety of wavelengths over a large portion of thespectrum (generally including visible light, near ultraviolet, and nearinfrared), and fluoresces or photoluminesces such that it emits light ina different range of wavelengths (generally infrared, although otherwavelengths may be used instead), at a lower energy (i.e., lowerfrequency or longer wavelength) than that of the absorbed light. Manybulk materials such as phosphorus are well known to fluoresce orphotoluminesce in this way. Semiconductors are particularly known foremitting light at their bandgap energy. Quantum confinement structuressuch as quantum wells, quantum wires, quantum dashes, and quantum dotsmay also be used as downconverters. Quantum confinement structures tendto have much higher efficiencies than semiconductors, i.e., they re-emita larger percentage of the energy they absorb.

Blackbody radiators may also be used as downconverters. Blackbodyradiators do not fluoresce, but they do absorb radiation (e.g., visiblelight) and re-emit it at longer wavelengths (e.g., infrared). Anexemplary form of a blackbody radiator may be as simple as a piece ofsteel or other metal possibly painted black to ensure broadbandabsorption. Other exemplary blackbody radiators that may be used in aTSOD filter may include dark-colored polymers and ceramics, honeycombsand other aerospace-derived “precision blackbody” type structures, oreven a simple coating of dark (ideally black) paint.

The downconverter may also be selected or designed to be“thermochromic.” In other words, the emission peak of the materialshifts with temperature. Many materials are known to have this property.Blackbody radiators shift their peak output wavelength according toWein's law. Semiconductor materials change their emission wavelength or“color” because temperature affects their interatomic spacing or latticeconstant and thus alters their effective bandgap. However, the effectivebandgap of quantum confinement structures (typically quantum wells,quantum wires, and quantum dots) is an inverse function of their size aswell as being a function of their composition and lattice constant.Thus, these structures are also thermochromic to some extent.

Another component of the TSOD filter is a notch filter or bandblockfilter, which is highly reflective to light in a particular range ofwavelengths, and highly transparent to other wavelengths within thespectrum of concern (typically near-UV, visible (VIS), and near-IRlight). Examples of UV/VIS/NIR bandblock filters include the distributedBragg reflector (DBR), which uses alternating layers of two transparentmaterials with different indices of refraction, and a rugate filter,which relies on a smoothly varying index of refraction from one face ofthe material to the other. Low emissivity coatings or films, forexample, coatings of indium tin oxide or other metal oxides may alsofunction as bandblock filters. As used herein, the terms “coating” and“film” may be used interchangeably and each is intended to encompass theother when applicable in any particular implementation. Sheets ofordinary glass or glass including borosilicate and many other types ofbandblock filters also exist which could be used in the TSOD filter withequal effectiveness, and which need not be elaborated here in order tofully convey the possible implementations of the design, functions, andutility of the TSOD filter. Because the bandblock filter is transparentto light outside of its stop band or stop bands, most of the incidentlight (e.g., sunlight) striking the TSOD filter passes directly throughthe bandblock filter with very little attenuation or reflection.Optical, infrared, and ultraviolet bandblock filters are well known inthe prior art, and need no elaboration here.

Many embodiments of the TSOD filter also include a thermochromic filteror attenuator, which is defined as any material, object, device, ormechanism whose transmissivity to incoming radiation (e.g., visiblelight) varies with temperature. For example, a thermotropic liquidcrystal, paired with a polarizer on each surface, may be used as anattenuator. This structure is generally transparent, but turns black(absorptive) above a threshold temperature, known as the clearing point,or in the presence of an electric field.

In many implementations the downconverter in its cold state absorbswhite light passing through the bandblock filter and emits monochromaticlight (e.g., near-infrared light at a wavelength of 2000 nm). This lightis emitted by the downconverter layer in all directions, but the vastmajority of this emission is basically normal or perpendicular to theTSOD filter. Fifty percent of this light passes inward and the other 50%passes outward. However, if the emission peak wavelength of thedownconverter falls within the stop band of the bandblock filter, thislight is reflected back through the downconverter again. Thus, the whitelight passing through the TSOD filter is converted tolower-frequency/longer-wavelength light, which is prevented fromescaping back out the way it came in. This light then passes through asecond bandblock filter, whose stop band has been selected such that themonochromatic light passes through it unattenuated in the cold state.

However, when the downconverter is above a threshold temperature, itsemission peak shifts such that it falls outside the stop bands of bothbandblock filters. In this case, since neither filter reflects themonochromatic light, half of the emissions of the downconverter radiateinto the device and the other half radiate out. Thus, the total energytransmitted by the device is half what it was in the cold state.

Finally, when the downconverter is above a second threshold temperature,its emission peak falls outside the stop band of the outer bandblockfilter, and inside the stop band of the inner bandblock filter. In thiscase, the monochromatic light which radiates out of the device ispermitted to leave, whereas the monochromatic light radiating into thedevice is reflected back out again. Thus, very little of the incidentlight energy striking the device is allowed to pass through it. Instead,it is converted to monochromatic light and then reflected away.

As a result, the TSOD filter exhibits three distinct behaviors: at lowtemperature it passes light energy through. At intermediate temperatureit reflects or radiates away approximately half of the light energy thatstrikes it, and transmits the other half. At high temperature, itreflects or radiates almost all of the incident light energy. Thus, theTSOD filter can be used to regulate the internal temperatures ofbuildings and other structures by controlling the amount of solarradiation they absorb.

The TSOD filter has particular, but not exclusive, application inregulating the temperatures of buildings by controlling the amount ofsolar radiation they absorb. In addition, it is possible to enhance theperformance of the TSOD filter by improving its cold-state lightabsorption or hot-state light rejection, or by decreasing its thermalconductivity. Furthermore, it is possible to increase the transparencyof the TSOD filter by adjusting the thickness, optical density, orarrangement of the downconverter layer (e.g., by alternating stripes orspots of downconverter material with transparent material). Thus, theTSOD filter may be functionally enhanced for certain applicationsthrough the addition of optional features such as fins, collimators,diffusers, attenuators, anti-reflection coatings, concentrating lenses,air gaps or vacuum gaps, or translucent thermal insulators including,but not limited to, foamed glass and silica aerogels.

Although the materials and structures of the TSOD filter may be rigid,there is no requirement for rigidity in order for it to perform thefunctions described herein. Furthermore, while the various components ofthe TSOD filter are shown and described as being attached or in directphysical contact, the TSOD filter will also function if the componentsare merely adjacent but physically separate. Thus, while the TSOD filtermay be embodied as a solid object (e.g., a brick, spandrel, or movablepanel) or group of solid objects (e.g., components affixed to an opticalworkbench), it can also be embodied as a flexible object such as, forexample, a tent material, blanket, curtain, or an appliqué film whichcan be applied to the surface of glass windows, spandrels, or glassblock building materials.

Although the maximum control over energy transport for the TSOD filteroccurs when the output wavelength of the downconverter is as large aspossible, the output wavelength can be selected to occur within thevisible spectrum for aesthetic reasons, or as a source of useful light.The output wavelength of the downconverter can further be chosen toprovide an emission wavelength for optimal catalysis of chemical orbiochemical reactions. For example, the emission wavelength could beoptimized to promote photosynthesis or sun tanning, or to produceparticular optical effects such as the excitation of a crystal, as in alaser. Because of the photoluminescent properties of the downconverter,the output of colored light from the TSOD filter is significantlybrighter than can be achieved by simply passing white light through acolored filter. In addition, it is possible to add a reflective “color”to the surface of the device, with minimal effect on its efficiency, byadding one or more additional bandblock filters to reflect particularwavelengths of light. The resulting optical properties do not closelyresemble those of any other building material.

FIG. 1 is a schematic, cross-section view of one embodiment of the TSODfilter 100 depicting a downconverter layer 102 sandwiched between twobandblock filters 101 and 103, and attached to a transparent substrate104. In the most general case the external light source will be whitelight, i.e., light with significant intensity across a significantbandwidth of the visible, near-UV and near-IR spectrum. In one exemplaryuse of the TSOD filter 100, the external light source is the sun.However, the TSOD filter 100 will also function when the external lightsource is not white, as for example the diffuse radiant energy of theblue sky.

Incoming light first passes through the outer bandblock filter 101. Inone embodiment, the bandblock filter has an extremely narrow stop band(bandwidth of 100 nm or less) in the infrared portion of the spectrum(i.e., wavelengths of 750 nm or greater). Exemplary forms of thebandblock filter 101 include a distributed Bragg reflector (DBR) orrugate filter. Both types of reflectors can be made from a variety ofmaterials. In exemplary implementations, the bandblock filter 101 may bea DBR composed of alternating layers of two different transparentpolymers, such as polystyrene (PS) and polymethyl methacrylate (PMMA). Aperson skilled in the art will understand that these layers can beformed by a variety of standard deposition techniques which need not beelaborated here. However, in exemplary implementations these layers maybe formed by spin-coating layers onto a substrate with liquidsconsisting of a single polymer dissolved in a solvent.

The portion of the incoming spectrum that falls within the stop band isreflected away by the bandblock filter 101. However, the bandwidth andcenter wavelength of the stop band will generally be selected such thatthese reflective losses are minimized. For example, only 2% of thesea-level solar spectrum occurs between the wavelengths of 2000 and 2200nm. Thus, a bandblock filter that reflected light in this range wouldnevertheless transmit up to 98% of incoming sunlight.

Once it has passed through the outer bandblock filter 101, the incominglight (e.g., sunlight) enters the downconverter 102, which is a deviceor material that absorbs high-energy light at a variety of wavelengthsand re-emits the light in a single, narrow band of wavelengths which arealways equal to or longer than the wavelengths absorbed. For example, anexemplary downconverter 102 with an emission peak at 2000 nm wouldabsorb light with a wavelength shorter than this, and re-emit the energyin a narrow, Gaussian band centered around 2000 nm. In general, thedownconverter 102 will be transparent to wavelengths longer than itsemission peak, so that when exposed to sunlight passing through theouter bandblock filter 101, the exemplary downconverter 102 would allowincoming radiation with a wavelength greater than 2000 nm—approximately7% of the total energy—to pass through unattenuated.

A variety of devices and materials exhibit this behavior, includingblackbodies which absorb and thermally re-radiate, and bulksemiconductors whose emission peak occurs at their bandgap energy.However, quantum confinement structures such as quantum wells, quantumwires, and quantum dots generally have higher optical efficiencies thanbulk semiconductors, so that a majority of incoming light is absorbedand converted, with only a small portion reflected away, transmittedthrough, or dissipated as waste heat. In some implementations, thedownconverter 102 consists of a plurality of quantum dot particlesembedded in a transparent polymer. However, the downconverter could alsobe a quantum well, an arrangement of quantum wires, a bulk material suchas a semiconductor, a doped or structured photonic material, or ablackbody absorber/radiator.

The structure, composition, manufacture, and function of quantum dotparticles generally is taught in U.S. Patent Application Publication No.2003/0066998 by Lee et al., which is hereby incorporated by reference asthough fully set forth herein. The structure, composition, manufacture,and function of exemplary quantum dot devices are taught in U.S. Pat.No. 5,889,288 to Futatsugi, which is hereby incorporated by reference asthough fully set forth herein. The structure, composition, andmanufacture of addressable quantum dot arrays is taught in U.S. Pat. No.6,978,070 to McCarthy et al., which is hereby incorporated by referenceas though fully set forth herein. It will be understood by a person ofordinary skill in the art that any quantum confinement structures ordevices employed in the TSOD filter as a downconverter may be ofdifferent design than those described by Lee et al., Futatsugi, andMcCarthy et al., while still performing the essential function ofoptical downconversion.

The action of incoming light is depicted in FIG. 1 for the cold state ofthe TSOD filter 100. The downconverter 102 absorbs incoming light andre-emits at a wavelength which is inside the stop band of the outerbandblock filter 101. Thus, any light emitted by the downconverter 102in the outward direction is reflected back into the device. However, inthe cold state the output wavelength of the downconverter 102 is outsidethe stop band of the inner bandblock filter 103. Thus, any light emittedby the downconverter in the inward direction is passed into and throughthe transparent substrate 104.

FIG. 2 is a schematic, cross-section view of the embodiment of FIG. 1,except that the action of incoming light is depicted for the hot stateof the TSOD filter 100. The downconverter 102 absorbs incoming light andre-emits at a wavelength which is outside the stop band of the outerbandblock filter 101. Thus, any light emitted by the downconverter 102in the outward direction is allowed to escape. However, in the hot statethe output wavelength of the downconverter 102 is inside the stop bandof the inner bandblock filter 103. Thus, any light emitted by thedownconverter 102 in the inward direction is reflected back, and doesnot reach or pass through the transparent substrate 104.

Thus, in its cold state the TSOD filter 100 transmits most of the lightenergy which strikes its outer surface, re-emitting it aslonger-wavelength light (e.g., infrared light) through the innersurface, whereas in the hot state the TSOD filter 100 re-emits thisenergy back through the outer surface, effectively rejecting it orreflecting it away. As a result, the TSOD filter 100 can be used toregulate the flow of light or radiant heat into a structure based on thetemperature of the TSOD filter 100.

From the above description, a person of ordinary skill in the art willrealize that in this embodiment, the transparent substrate 104 ispresent only for reasons of structural support and convenience. Thiscomponent may be deleted without significantly altering the function ofthe TSOD filter 100. Alternatively, the transparent substrate 104 couldbe placed on the outer surface of the TSOD filter 100 rather than theinner surface, or transparent substrates 104 could be placed on bothsurfaces, or even inserted between one or more of the functional layersof the TSOD filter 100, without significantly altering its function.Furthermore, if the transparent substrate 104 is located on the insidesurface of the device as shown in FIGS. 1 and 2, it need not betransparent to all wavelengths, and can in fact be a longpass,shortpass, or bandpass filter as long as the output wavelength of thedownconverter 102 falls within the passband of the substrate 104. Inother words, the substrate 104 need only be transparent to thewavelengths emitted by the downconverter 104 in its cold state. However,for convenience and cost it will generally be simpler to use an ordinarytransparent material such as glass or acrylic as the substrate 104.

FIG. 3 is a diagram of light intensity vs. wavelength depicting theemission spectrum of the external light source, the fluorescencespectrum or photoluminescence spectrum of the downconverter 102 in coldand hot states, and the reflection spectrum of the two bandblock filters101 and 103 of an implementation of a TSOD filter 100. As thetemperature of the TSOD filter 100 varies, the emission peak of thedownconverter 102 moves back and forth, falling within the reflectionband of the outer bandblock filter 101 at low temperature, and withinthe reflection band of the inner bandblock filter 103 at hightemperature. At intermediate temperatures the emission band may falloutside the reflection bands of both filters 101, 103. However, under nocircumstances does it fall within the reflection bands of both filters.Thus, depending on the temperature of the TSOD filter 100 the lightemitted by the downconverter 102 is either reflected inside, reflectedoutside, or radiated equally in both directions.

Although for convenience the inner filter 103 is described as abandblock filter, it can be replaced with a longpass filter whose stopband has the same upper cutoff wavelength as the equivalent bandblockfilter, but whose lower cutoff wavelength extends, in principle, all theway to zero. This will not affect the essential functioning of the TSODfilter 100, although it will prevent the TSOD filter 100 fromtransmitting wavelengths too long to be absorbed by the downconverter102.

Also, it is possible to design an embodiment of the TSOD filter 100wherein the downconverter 102 is not thermochromic, i.e., it exhibits asingle emission peak for all temperatures. In this case, the outerbandblock filter 101 and inner bandblock filter 103 must bethermochromic instead. This will occur, for example, in a distributedBragg reflector made from materials with a very high coefficient ofthermal expansion. Such effects may be difficult to harness except oververy large temperature ranges, but such an embodiment may be appropriatefor certain applications.

FIG. 4 is a schematic, cross-section representation of anotherembodiment of the TSOD filter 100, in which there are gaps 105 in thedownconverter 102 to allow some white light from the external source topass through the TSOD filter 100 without modification. These gaps 105may take the form of holes or stripes, or alternatively thedownconverter material itself may be applied in stripes or spots. Itshould be noted that if the downconverter 102 consists of a liquid orparticulate material (e.g., a plurality of quantum dots suspended in atransparent polymer), this material could be dissolved into a solventand “painted on” through a stencil punched with a plurality of holes.Then, as with any other paint, the solvent would be allowed toevaporate, leaving behind the downconverter material in a pattern ofspots, or any other pattern the stencil might hold. However, a person ofordinary skill in the art will understand that there are numerousalternate methods for fashioning the gaps 105 that need not beelaborated here. This embodiment may be useful, for example, in windowswhich are required to offer a relatively clear view from inside tooutside. In this case, the attenuation or obstruction of thedownconverter 102 would be similar to looking through a normal windowscreen.

The use of a downconverter 102 with gaps 105 in place of a uniformdownconverter increases the transmission of energy through the TSODfilter 100 under all conditions, and thus reduces the ability of theTSOD filter 100 to reject radiant heat in its hot state. However, thisarrangement may be advantageous under circumstances where cold-stateperformance is more important than hot-state performance.

FIG. 5 is a schematic, cross-section representation of an additionalembodiment of the TSOD filter 100 in which a number of optionalcomponents have been added in order to improve the performance andaesthetics of the device. The functioning of the outer bandblock filter101, downconverter 102, inner bandblock filter 103, and transparentsubstrate 104 is identical to that described for FIGS. 1 and 2. However,each of the optional components serves a new function which affects theperformance and/or aesthetics of the overall device. These optionalcomponents all operate independently of one another, i.e., none of themdepend on any other optional component in order to perform its function.For convenience, this embodiment will be described as shown in FIG. 5with all of the optional components in place simultaneously, but areader of ordinary skill will understand that with some optionalcomponents present and some not, the possible permutations are extremelynumerous and need not be discussed individually.

Before light enters the outer bandblock filter 101, it first passesthrough a set of fins 108. In the simplest embodiment, these fins 108are parallel, horizontal strips of an opaque, reflective, or translucentmaterial that allow incoming light to pass through unaffected whenincident at an angle which is perpendicular or nearly perpendicular tothe surface of the TSOD filter 100, but restrict, block, absorb,reflect, or attenuate light which is incident at an angle closer toparallel to the surface of the device. In the case where the incominglight is sunlight and the TSOD filter 100 is oriented vertically (e.g.,as part of a wall or window), this arrangement will allow more light toenter when the sun is low in the sky (e.g., during the winter), andallow less light to enter when the sun is high (e.g., in the summer).Thus, the TSOD filter 100 has an improved ability to exclude radiantheat from outside in hot weather. A person of ordinary skill in the artwill understand that these fins could assume a variety of other formswithout altering their essential function. They could be of differentshape than shown here, including opaque wedges and cylinders, ortransparent lenses of a variety of shapes. Alternatively, a diffractiongrating, Fresnel lens, or other optics attached to or embossed on thesurface of the TSOD filter 100 could be used to bend incoming light suchthat only photons entering the device at particular angles are permittedto reach the downconverter 102.

After passing through the fins 108, the incoming light next enters acollimator 107. The purpose of the collimator 107 is to “straighten” theincoming light so that it is all traveling perpendicular to the layersof the TSOD filter 100 while it remains within the collimator 107. Fordownconverters 102 or bandblock filters 101, 103 which incorporateperiodic crystal-like arrangements of microscopic grains, cells,particles, or layers, the incidence angle may have a significant effecton optical properties, and the addition of a collimator 107 can help toreduce such effects where they are not wanted. Exemplary forms of acollimator 107 may include an arrangement of hollow cylinders, fusedfiber optics, or the mineral ulexite (also known as “TV stone”),although other forms also exist.

After passing through the collimator 107, the incoming light enters anattenuator 106. The simplest form of attenuator 106 is a neutral-densityfilter that blocks a percentage of the incoming light at allwavelengths, thus reducing the intensity of the light withoutsignificantly affecting its spectrum. The addition of such an attenuator106 will reduce the transmission of light energy through the TSOD filter100 in all temperature states, thus limiting the ability of the TSODfilter 100 to direct radiant heat in the cold state. This may beadvantageous in applications where hot-state performance is moreimportant than cold-state performance. The skilled reader will note thatfor some applications it may be advantageous to place other components,such as the attenuator 106 or downconverter 102 internal to thecollimator 107, although it is not shown this way in FIG. 5.

Alternatively, in other applications it may be more favorable to use anattenuator 106 with non-neutral density, i.e., a color filter. Forexample, a shortpass filter could be used to reflect away wavelengths oflight too long to be absorbed and reradiated by the downconverter 102,since these wavelengths cannot be controlled by the temperature-basedswitching of the TSOD filter 100. The attenuator 106 may also be abandblock filter such as a distributed Bragg reflector or rugate filterwhich reflects light within a narrow range of wavelengths. This willslightly decrease the amount of energy available to the downconverter102, which may be advantageous for certain applications, and it willalso provide a reflective “color” for the outside surface of the TSODfilter 100, which may serve an aesthetic purpose where the color fallswithin the visible spectrum.

In still other circumstances, the attenuator 106 may be aphotodarkening, photochromic, electrodarkening, or electrochromicmaterial or device, plus supporting hardware that may be required tooperate it (e.g., a photovoltaic cell, a temperature sensor, and acontrol circuit to lighten and darken an electrolyte-basedelectrochromic filter). The attenuator 106 may even be a mechanicalattenuator such as a shutter, a curtain, or a set of louvers, plus anysensors, power sources, and control systems required to operate it(e.g., a temperature-sensitive bimetallic coil such as those found incertain types of thermometers). It is also possible to include multipleattenuators of various types within the same TSOD filter 100.

In one embodiment the attenuator 106 may be a thermochromic orthermodarkening material with transmission, absorption, and/orreflection spectrums that are a function of temperature. Exemplary formsof thermochromic material include zinc oxide (which changes from clearto yellow when heated and reflects light), liquid crystals (which can beformulated to absorb or reflect a percentage of the incident visiblelight above a given threshold temperature), and tungsten-doped vanadiumoxides (which reflects light above a threshold temperature, determinedin part by the percentage of tungsten in the composition of thematerial).

Once the incoming light has been downconverted to monochromatic light inthe downconverter 102, and has passed through the inner bandblock filter103 and transparent substrate 104, the light then passes through a colorfilter 109 whose purpose is to provide a reflective color to theinterior surface of the TSOD filter 100 for aesthetic purposes. In oneform, the color filter 109 may be a bandblock filter with a stop bandthat falls within the visible spectrum. However, the color filter 109may also be a longpass, shortpass, or bandpass filter, or stacked (i.e.,additive) combination of filters. As long as the stop band or stop bandsof the color filter 109 do not include the output wavelengths of thedownconverter 102, the functioning of the TSOD filter 100 will not beaffected, and the ability of the device to transmit energy in its coldstate or reject energy in its hot state will not be reduced.

Another optional component is an external reflector 112 to increase thelight-gathering area of the TSOD filter 100, in the same way that atelescope mirror increases the light-gathering area of the objective.The external reflector 112 could take virtually any shape and hold avariety of external positions too numerous for elaboration here. Thesimplest exemplary form of the external reflector 112 is an ordinarymirror placed on the ground, reflecting light up into the TSOD filter100. Such a component is arguably an external enhancement or adjunct tothe TSOD filter 100 rather than a component of the device itself, butsome embodiments could include such a reflector 112 as an integralcomponent of the TSOD filter 100.

Another optional enhancement, not pictured in FIG. 5, is to applyantireflection coatings to the surfaces of any or all of the componentsin the TSOD filter—most particularly those exposed to outside air or tointernal air gaps, gas gaps (e.g., argon or krypton filled gaps), orvacuum gaps, or other interfaces where the refractive index of onematerial is significantly different from the refractive index of itsneighbor. The use of the term “air gap” herein is meant to include airgaps, gas gaps, and vacuum gaps collectively and should be interpretedas such unless explicitly stated otherwise. In general, such coatingsare microscopically thin, and vary widely in composition depending onthe exact application and on the refractive indices of the two materialsbeing matched. This technique is well described in the prior art andneed not be elaborated here.

FIG. 6 is a schematic representation of a further embodiment of the TSODfilter 100, in which the white light from the external light source ispassed through a concentrating lens 110 before striking the outerbandblock filter 101 and downconverter 102. The purpose of theconcentrating lens 110 is to project the incoming light from a largearea of lens onto a small area of bandblock filter 101 and downconverter102, either to increase optical efficiency by locally increasing theintensity of the light, or to decrease material requirements by allowingsmaller bandblock filters 101, 103 and downconverter 102. This lens 110could assume a variety of forms—from standard concave and convex designsto spherical, conical, cylindrical, or other shapes designed toconcentrate the light in different ways, or on different regions, or todifferent extents, and could be a complex series of lenses, as in acamera or telescope.

Because concentrated light (e.g., concentrated sunlight) is often a firehazard or injury hazard, this embodiment may also include a diffuser orde-concentrating lens 111 to prevent light from exiting the TSOD filter100 in a concentrated beam. Like the concentrating lens 110, thediffuser 111 could assume a variety of forms, although these are lesslimited than the possible forms of the concentrating lens sincede-concentrating or diffusing light is a less demanding application.However, if the diffuser 111 is not included, the TSOD filter 100 hasapplications as an infrared beam generator, similar in some respects toa laser (though not coherent), which could be used for example inswitchable cooking and heating devices such as water heaters thatoperate over a modest distance.

A reader of ordinary skill will note that the TSOD filter 100 in any ofthe aforementioned embodiments could function in a degraded capacitywith one of its bandblock filters deleted. With the outer bandblockfilter 101 missing, the TSOD filter 100 would still function normally inits hot and intermediate states, but would not capture energy aseffectively in its cold state. That is, the cold state would behave thesame as the intermediate state, capturing approximately half theincident energy and radiating the rest of it back outside. Such anembodiment might be easier or less expensive to build and deploy in hotclimates, where cold-state performance is not a significant issue.

With the outer bandblock filter 101 present but the inner bandblockfilter 103 missing, the TSOD filter 100 would function normally in thecold and intermediate states, but would not reject light energy aseffectively in its hot state. In other words, the hot state would behavethe same as the intermediate state, radiating away about half theincident energy, while allowing the other half to pass through thedevice. Such an embodiment might be easier or less expensive to buildand deploy in cold climates, where hot-state performance is not asignificant issue. In both cases, the switchability of the device can beimproved when a thermochromic attenuator 106 is included as part of theTSOD filter 100.

In an exemplary embodiment, the concentrating lens 110 andde-concentrating lens 111 may be made of a clear, flat polymer such asPMMA, etched with a Fresnel pattern, while the bandblock filters 101,103, and 109 may be DBRs composed of multiple layers of transparentpolymers (e.g., PS and PMMA), and the downconverter 102 may be made ofsemiconductor quantum dot particles (e.g., cadmium telluridenanoparticles) suspended in a transparent polymer such as PS. The fins108 may be made from a white, reflective polymer; the collimator 107 maybe made from fused optical fibers; and the transparent substrate 104 maybe made from transparent polymer. The attenuator 106 may be made from afilm of tungsten-doped vanadium dioxide. The entire TSOD filter 100 mayform a rigid panel or flexible appliqué film which can be affixed totransparent building materials such as windowpanes, glass spandrels, andglass blocks, either as a retrofit to existing structures or as aseparately installable building structure.

Alternatively, the concentrating lens 110 and de-concentrating lens 111may be combined as a single component, such as for example a transparentrod or optical fiber that is thin in the middle and flared at both ends.Such an arrangement makes it possible to replace transparent components(e.g., the substrate 104) with opaque components that are completelypenetrated by an array of optical fibers. This may be done for reasonsof cost, improved insulation, structural strength, or for other reasons.

FIG. 7 is a schematic representation of an additional embodiment of theTSOD filter 100, wherein the inner bandblock filter 103 has been deletedand the outer bandblock filter 101 has been combined with thetransparent substrate 104 as a single component. In addition, thedownconverter 102 is located at or near the “building interior” surfaceand a thermochromic attenuator 106 is located at or near the “buildingexterior” or sunward face of the device.

For purposes of this document, the term “thermochromic attenuator”should be understood to include not only passive devices which changecolor, opacity, attenuation or reflectivity in response to temperature,but also complex devices with multiple components. For example, anelectrochromic attenuator, combined with a power supply, control system,and temperature sensor would serve the same function as a naturallythermochromic material and can be used interchangeably with it, althoughthe device is not shown that way in FIG. 7.

In this embodiment, white light enters the attenuator 106, where it isabsorbed or reflected in the hot state, so that minimal or no radiationis allowed into the interior of the TSOD filter 100. In the cold state,the thermochromic attenuator 106 is more transparent, so radiation(e.g., sunlight) is allowed to pass through the transparent substrate104 and strike the downconverter 102.

Unlike other embodiments, the thermochromic downconverter 102 in thisembodiment converts incoming radiation to a longer infrared wavelength(typically >5000 nm) such that at low temperatures its output radiationis reflected by the substrate 104. This allows the substrate 104 to actas a bandblock filter 101, so that the entire output of thedownconverter 102 is reflected away from the interior of the TSOD filter100 and out through the inner surface. At higher temperature, the outputof the downconverter 102 shifts toward shorter frequencies, until athreshold temperature is crossed and the emitted radiation begins toexceed the cutoff wavelength of the substrate 104, and thus to fallwithin its passband. For typical transparent glasses and plastics, thispassband occurs between the wavelengths of approximately 200 nm andapproximately 5000 nm. In other words, the substrate is transmissive toradiation between these two wavelengths, and opaque (usually reflective)to radiation of longer or shorter wavelength. However, in the hot statethe attenuator 106 will limit or prevent radiation from reaching thedownconverter 102 and thus being re-emitted.

One advantage of this arrangement is that the “building interior”surface of the downconverter 102 can be painted any color, usingvirtually any paint chemistry, without altering the essential functionof the downconverter 102. This is analogous to painting an old-fashionedsteam radiator. Alternatively, the interior surface could be coveredwith a plaster, stucco, or other treatment, with or without a pigment.

In addition, this embodiment includes an optional layer of transparentinsulation 113. In one exemplary embodiment, this insulation may consistof silica aerogel, possibly encapsulated by other transparent materials.However, other transparent materials can also be used, including but notlimited to, glass or polymer beads or hollow spheres, bubble wrap, orsequentially stacked sheets of transparent material, whether rigid orflexible, that disrupt the conduction and convection of heat whilehaving little effect on the radiant transmission of visible and nearinfrared light.

FIG. 8 is a schematic representation of still another embodiment of theinvention, wherein the transparent substrate 104 and the outer bandblockfilter 101 have been combined into a single component as in FIG. 7, andin addition the downconverter 102 and the thermochromic attenuator 106have been combined into a single component.

In this embodiment, the attenuator 106 may take the form of athermochromic or electrochromic material that is reflective (e.g.,white, metallic, or mirrored) in the hot state and absorptive (e.g.,black) in the cold state. For exemplary purposes, FIG. 8 shows theattenuator 106 as an electrochromic material whose color is controlledby a power supply 115, a temperature sensor 116, and a controller 117.In one implementation the power supply 115 is a photovoltaic cell, thetemperature sensor 116 is a solid-state electronic sensor such as athermocouple, the attenuator 106 and downconverter 102 consists of atwo-color, high-contrast electrochromic material such as “electronicpaper,” and the controller 117 is a circuit board connected to the powersupply 115 and temperature sensor 116 by wires. Methods for sensingtemperature, regulating photovoltaic energy, and controlling the colorof electrochromic materials, are well understood in the prior art andneed not be elaborated here.

When white light (e.g., sunlight) enters the transparent substrate 104,it is passed through to the attenuator 106, which reflects it in the hotstate so that it exits the device through the transparent substrate 104.In the cold state, incoming radiation is absorbed by the attenuator 106,which then heats up and re-emits the energy as long-wavelength infrared.Thus, the attenuator 106 also serves as a form of downconverter 102.

When this energy is re-emitted by the downconverter 102 in the coldstate as infrared, the energy then reflects off the transparentsubstrate 104, as in FIG. 7 and is directed out through the “buildinginterior” face of the TSOD filter 100. Alternatively, the energy maystrike an optional mirror or broadband reflector 114, which reflects itback into the downconverter 102 again. The reader will note that in thiscircumstance, infrared light is not able to escape from thedownconverter layer, and all interior heating is accomplished throughconduction alone. In this case, the geometry and composition of thedevice will generally be designed such that conduction occurspredominantly in the desired (into the building) direction, with minimalleakage in the other direction.

In an alternative embodiment, the attenuator 106 is absorptive in thecold state but transmissive (e.g., transparent or translucent) in thehot state. In this embodiment, in the hot state the incoming white lightpasses through the attenuator 106 and strikes a mirror 114, whichreflects it back out through the transparent substrate 114. In the coldstate, the light is downconverted and then trapped in the downconverteras described above. However, this configuration is particularlyeffective at rejecting heat, i.e., directing radiant energy to theoutside in the hot state.

FIG. 8 also shows an additional optional component: an energy-storingmaterial 118, which absorbs heat from the downconverter 102 andre-releases it over a longer period of time. In one embodiment, theenergy-storing material 118 is a phase-change material with a very largeheat of fusion, with a melting temperature selected to be close to roomtemperature, or to some other desired temperature. Suchmaterials—generally waxes or specialized salts—hold a constanttemperature unless completely liquefied or completely solidified, andare thus good for smoothing out large changes in incoming radiation. Anexemplary phase change wax that may be used for energy storage is commonparaffin. Glauber's salt (sodium sulfate decahydrate) is a typical saltthat may be used for energy storage in some implementations.

FIG. 9 is a schematic representation of an additional embodiment of theTSOD filter 100, wherein the thermochromic attenuator 106 has beenplaced on the inside surface, rather than the outside surface, of thetransparent substrate 104. In addition, the bandblock filter 101 and thetransparent insulator 113 have been combined as a single component. Thislayer separates the attenuator 106 from the building interior, so thatenergy absorbed by the attenuator in the hot state is not re-radiatedinto the building interior. Ideally, the insulation value of theinsulator 113 is significantly higher than that of the substrate 104, sothat conductive heat transport out of the attenuator goes mainly to thebuilding exterior, rather than the building interior. This arrangementalso makes it convenient to employ the device as a retrofit forpre-existing transparent building components, including windows andglass block walls, and glass spandrels, turning them into active,heat-regulating wall elements that may or may not allow visible lightthrough.

In the cold state, once incoming light has passed through the substrate104, the attenuator 106, and the insulator 113/band reflector 101, itstrikes the downconverter 102, where it is converted to infrared,re-radiated, and reflected off the band reflector 101, such that theenergy is directed toward the building interior rather than the buildingexterior. It is also possible to have an air gap between thedownconverter 102 and the insulator 113/band reflector 101, althoughthis is generally less efficient because much of the energy released bythe downconverter goes to heating this air gap rather than the buildinginterior. In the extreme case, objects and surfaces within the buildinginterior itself can serve as downconverters, although they will beinefficient in this role unless they are by nature highly absorptive andradiative. However, in this case the air gap and the air of the buildinginterior are one and the same.

FIG. 10 depicts an exemplary implementation of a TSOD filter 100 in theform of a glass spandrel for use in exterior cladding of buildings.Spandrels are often sections with a glass exterior (although opaquematerials may be used as well) that span sections of a building betweenwindows and appear as windows as well, but in fact enclose areas of thebuilding housing mechanicals (e.g., the space above drop ceilings andthe space behind floor radiators below windows. As shown in FIG. 10, thespandrel layers starting from the exterior of a building may include apane of ordinary window glass 104 adjacent a thermochromic attenuator106. An interior surface of the thermochromic attenuator 106 may becoated with a low-emissivity film 101, e.g., indium tin oxide, that actsas a bandblock. This low emissivity coating 101 is optional to thefunctionality of the TSOD filter 100 in this spandrel embodiment, butmay be extremely helpful as described below.

A blackbody radiator, e.g., a steel plate painted black, may serve asthe downconverter 102 and may be spaced apart from the thermochromicattenuator 106 to form an air gap 113. The air gap 113 serves as atransparent insulator as discussed in prior embodiments. The firstlow-emissivity coating 101 helps reduce heating of the air gap 113 byblocking much of the infrared energy within the incident light fromreaching the air gap 113 and thus improving the insulating capacity ofthe air gap 113. A second low-emissivity coating 103, e.g., indium tinoxide, covers the exterior surface of the downconverter 102 to act as abandblock. As shown in FIG. 10, a second thermochromic attenuator 119may optionally be inserted into the air gap 113 between thedownconverter 102 and the first thermochromic attenuator 106. If used,the second thermochromic attenuator should be in direct or indirectthermal contact with the energy-storing material 118 in order to respondto changes in the temperature of the energy-storing material 118.

A backplate 120 forms the interior surface of the spandrel. Thebackplate 120 may be another plate of glass or a sheet of plastic ormetal. The backplate 120 is spaced apart from the downconverter 102 toform a gap. The gap is filled with an energy storing phase-changematerial 118, for example, a wax or a salt, that is present to store andrelease thermal energy. The interior surface of the backplate 120 mayalso be covered with an aesthetic surface treatment, for example, paintor thin stucco.

In operation, the first thermochromic attenuator 106 adjacent the glass104 at the exterior side of the spandrel may be chosen to have atransition temperature at which the first thermochromic attenuator 106blocks incident light when the ambient outside temperature is above0°-10° C., for example. In this way, the incident light does not passthrough the spandrel to heat the building on a moderately warm or hotday, thus reducing the cooling requirements for the building andconserving energy. However, when the ambient outside temperature isbelow 0°-10° C., for example, the thermochromic attenuator 106 passesthe incident light through the TSOD filter 100 to the downconverter 102.The downconverter 102 absorbs the incident light and emits energy atinfrared wavelengths. The second low-emissivity coating 103 reflects themajority of the infrared energy emitted from the downconverter 102 andprevents this energy from exiting the TSOD filter 100 at the exterior ofthe building. Thus, most of the infrared energy emitted by thedownconverter 102 is directed to the energy storing phase changematerial 118.

The phase change material 118 stores the infrared energy as thermalenergy, which is transferred to the interior of the building viaconduction through the backplate 120. As long as the interiortemperature of the building is less than the temperature of the phasechange material 118, thermal conduction will transfer heat from thephase change material 118 to the building. However, once the phasechange material 118 fully melts, it is unable to absorb any additionalthermal energy and will radiate any excess thermal energy to thebuilding as the air gap 113 effectively forecloses significant thermaltransfer to the exterior of the building. This thermal runaway conditionmay be acceptable until the interior of the building reaches a desiredroom temperature. However, without further control of the TSOD filter100 the phase change material 118 in thermal runaway could overheat thebuilding.

The second thermochromic attenuator 119 may optionally be provided toregulate the potential thermal runaway and prevent overheating of thebuilding. In such an implementation, the transition temperature of thesecond thermochromic attenuator 119 may be chosen to be approximatelyroom temperature, e.g., 20° C. Thus, below room temperature the secondthermochromic attenuator 119 passes all incident light through to thedownconverter 102 to heat the phase change material 118. Note that dueto the air gap 113, the second thermochromic attenuator 119 is insulatedfrom the ambient outside temperature and will only be heated throughthermal conduction from the phase change material 118 and the interiorof the building. However, once the interior of the building reaches roomtemperature and thus the phase change material 118 and the downconverter102 are also at room temperature, the second thermochromic attenuator119 transitions to block the incident light from reaching thedownconverter 102, even in the cold state as shown in FIG. 10, and thusprevents thermal runaway and excess heating of the interior of thebuilding.

While several exemplary embodiments of the TSOD filter are depicted anddescribed herein, it should be understood that the TSOD filter is notlimited to these particular configurations. Optional components may beadded or moved to suit the needs of a particular application or aparticular manufacturing method, and degraded forms of some embodimentscan be produced by, for example, deleting components such as onebandblock filter in an embodiment which normally contains two. A widevariety of other materials can be used to manufacture the TSOD filter,including, metals, ceramics, glasses, nanostructured and microstructuredphotonic materials, and even ices, liquids, and vapors. The TSOD filtermay include features designed to enhance its thermal insulationproperties including but not limited to air gaps, gas gaps, vacuum gaps,foams, beads, fiber pads, or aerogels. It may be thick and rigid enoughto serve as a structural component of vehicles or building walls. It maybe wrapped around or formed upon complex surfaces. It may beaesthetically enhanced with color or it may be camouflaged to resemblemore conventional building materials. Thermochromic pigments may beadded to certain surfaces to indicate when they are hot or cold.Mechanical enhancements may be added to reorient components, either toface them toward or away from incoming light, or to alter theirwavelength response or apparent thickness. The exact arrangement of thevarious layers can be different than is depicted here, and (depending onthe materials and wavelengths selected) different layers can be combinedas single layers, objects, devices, or materials (for example, anenergy-absorbing phase-change material that is also a thermochromicattenuator or band reflector), without altering the essential structureand function of the TSOD filter.

Although the description above contains many specificities, these shouldnot be construed as limiting the scope of the invention but ratherconstrued as merely providing illustrations of certain exemplaryembodiments of this invention. There are various possibilities formaking the device of different materials, and in differentconfigurations. For example, the structure could be inflatable or couldbe optimized for use underwater or in vacuum instead of in normal air.The bandblock filters could be thermochromic, either in place of or inaddition to any thermochromic properties of the downconverter. Althoughupconversion is generally less efficient than downconversion, anupconverter could be used in place of the downconverter for someapplications, particularly if advances in upconversion technologyimprove the efficiency of the upconversion process. Such embodiments areexplicitly claimed as part of the present invention.

Numerous other variations exist which do not affect the core principlesof the operation of the TSOD filter. For example, downconverter could becomposed of a single material such as phosphorus or silicon, could becomposed of a compound semiconductor such as cadmium telluride, or couldbe composed of doped, nanostructured, or microstructured materialsincluding, but not limited to, custom photonic crystals. Thedownconverter could be monocrystalline, polycrystalline, or amorphous.It could be a quantum well, an arrangement of quantum wires, or a“crystal” composed of regularly spaced quantum dots. It is evenconceivable that the downconverter could be a liquid, a vapor, or asuspension of nanoparticles, nanowires, nanoflakes, etc., in some mediumother than a solid polymer. One or more bandblock filters could benon-planar (e.g., parabolic) in shape, or other shaped reflectors orsimilar devices could be incorporated, to help concentrate incominglight from a variety of angles.

The use of the present invention as a thermally-regulating buildingmaterial may be enhanced by careful positioning of the TSOD filter, forexample by placing it under the eave on the south face of a house sothat the TSOD filter is in full sunlight during winter days and isshadowed by the eave on summer days when the sun is higher in the sky.Alternatively, the TSOD filter can be used in place of traditionalskylights, or as a panel or appliqué affixed to ordinary glass windowsor glass blocks.

Although various embodiments of this invention have been described abovewith a certain degree of particularity, or with reference to one or moreindividual embodiments, those skilled in the art could make numerousalterations to the disclosed embodiments without departing from thespirit or scope of this invention. All directional references e.g.,proximal, distal, upper, lower, inner, outer, upward, downward, left,right, lateral, front, back, top, bottom, above, below, vertical,horizontal, clockwise, and counterclockwise are only used foridentification purposes to aid the reader's understanding of the presentinvention, and do not create limitations, particularly as to theposition, orientation, or use of the invention. Connection references,e.g., attached, coupled, connected, and joined are to be construedbroadly and may include intermediate members between a collection ofelements and relative movement between elements unless otherwiseindicated. As such, connection references do not necessarily imply thattwo elements are directly connected and in fixed relation to each other.It is intended that all matter contained in the above description orshown in the accompanying drawings shall be interpreted as illustrativeonly and not limiting. Changes in detail or structure may be madewithout departing from the basic elements of the invention as defined inthe following claims.

1. A thermally switched optical filter comprising a substrate; adownconverter supported by the substrate, wherein the downconverterabsorbs incident light of a broad bandwidth and emits light at anemission wavelength substantially or entirely longer than wavelengths ofthe broad bandwidth; and a first bandblock filter supported by thesubstrate, wherein the first bandblock filter blocks the emitted lightwhen the temperature of the thermally switched optical filter is in afirst range and passes the emitted light when the temperature of thethermally switched optical filter is in a second range.
 2. The thermallyswitched optical filter of claim 1 further comprising an outer surfacethat receives the incident light; and an inner surface, wherein when thefirst range is a low temperature compared to the second range, theemitted light exits the thermally switched optical filter from the innersurface.
 3. The thermally switched optical filter of claim 1 furthercomprising an outer surface that receives the incident light; and aninner surface, wherein when the first range is a high temperaturecompared to the second range, the emitted light exits the thermallyswitched optical filter from the outer surface.
 4. The thermallyswitched optical filter of claim 1 further comprising an outer surfacethat receives the incident light; and an inner surface, wherein when thetemperature of the thermally switched optical filter is between thefirst range and the second range, the emitted light exits the thermallyswitched optical filter from both the inner surface and the outersurface.
 5. The thermally switched optical filter of claim 1 furthercomprising a second bandblock filter supported by the substrate, whereinthe second bandblock filter passes the emitted light when thetemperature of the thermally switched optical filter is in the firstrange and blocks the emitted light when the temperature of the thermallyswitched optical filter is in the second range.
 6. The thermallyswitched optical filter of claim 5 further comprising an outer surfacethat receives the incident light; and an inner surface, wherein when thefirst range is a low temperature compared to the second range, theemitted light exits the thermally switched optical filter from the innersurface.
 7. The thermally switched optical filter of claim 5 furthercomprising an outer surface that receives the incident light; and aninner surface, wherein when the first range is a high temperaturecompared to the second range, the emitted light exits the thermallyswitched optical filter from the outer surface.
 8. The thermallyswitched optical filter of claim 5 further comprising an outer surfacethat receives the incident light; and an inner surface, wherein when thetemperature of the thermally switched optical filter is between thefirst range and the second range, the emitted light exits the thermallyswitched optical filter from both the inner surface and the outersurface.
 9. The thermally switched optical filter of claim 5 furthercomprising a longpass filter supported by the substrate, wherein thelongpass filter passes the emitted light when the temperature of thethermally switched optical filter is in the first range and blocks theemitted light when the temperature of the thermally switched opticalfilter is in the second range.
 10. The thermally switched optical filterof claim 5, wherein each of the first bandblock filter and the secondbandblock filter is thermochromic.
 11. The thermally switched opticalfilter of claim 1, wherein the downconverter is a blackbody radiator.12. The thermally switched optical filter of claim 1, wherein thedownconverter is a fluorescent, phosphorescent, or photoluminescentmaterial.
 13. The thermally switched optical filter of claim 1, whereinthe downconverter comprises a plurality of quantum confinement devicesembedded in a transparent material.
 14. The thermally switched opticalfilter of claim 1, wherein the downconverter defines openings throughwhich the incident light passes without absorption.
 15. The thermallyswitched optical filter of claim 1, wherein the substrate istransparent.
 16. The thermally switched optical filter of claim 1,wherein the downconverter is thermochromic and the emission wavelengthis variable depending upon a temperature of the downconverter.
 17. Thethermally switched optical filter of claim 1, wherein the firstbandblock filter is thermochromic.
 18. The thermally switched opticalfilter of claim 1, wherein the emission wavelength of the downconverteroccurs in the visible spectrum.
 19. The thermally switched opticalfilter of claim 1, wherein the emission wavelength of the downconverteroccurs in the infrared spectrum.
 20. The thermally switched opticalfilter of claim device of claim 1, wherein the emission wavelength ofthe downconverter is selected for optimal catalysis of a chemicalreaction or a particular optical effect.
 21. The thermally switchedoptical filter of claim 1, wherein the substrate is a flexible fabric orpolymer sheet.
 22. The thermally switched optical filter of claim 1,wherein the substrate is glass or a transparent or translucent rigidpolymer material.
 23. The thermally switched optical filter of claim 1further comprising an external reflector that is positioned to directthe incident light toward an outer surface of the thermally switchedoptical filter.
 24. The thermally switched optical filter of claim 1further comprising a plurality of fins positioned at an angle to anouter surface of the thermally switched optical filter that partiallyrestrict, block, absorb, reflect, or attenuate the incident light fromreaching the outer surface.
 25. The thermally switched optical filter ofclaim 1 further comprising an attenuator that blocks a percentage of theincident light across the broad bandwidth.
 26. The thermally switchedoptical filter of claim 25, wherein the attenuator comprises athermochromic or thermotropic liquid crystal device.
 27. The thermallyswitched optical filter of claim 25, wherein the attenuator comprises athermochromic, optically reflective material.
 28. The thermally switchedoptical filter of claim 27 further comprising a control system; a powersupply connected with the control system and the attenuator; and one ormore sensors connected with the control system; wherein the attenuatorfurther comprises an electrochromic material that is activated by thecontrol system based upon feedback from the sensors to pass or block theincident light from reaching the downconverter.
 29. The thermallyswitched optical filter of claim 27 further comprising a control system;a power supply connected with the control system and the downconverter;and one or more sensors connected with the control system; wherein thedownconverter further comprises an electrochromic material that isactivated by the control system based upon feedback from the sensors toalter the emission wavelength of the emitted light.
 30. The thermallyswitched optical filter of claim 25, wherein the attenuator comprises athermochromic, infrared reflective material.
 31. The thermally switchedoptical filter of claim 1 further comprising a collimator that orientsthe incident light toward an outer surface of the thermally switchedoptical filter.
 32. The thermally switched optical filter of claim 1further comprising a concentrating lens that focuses the incident lighton an outer surface of the thermally switched optical filter.
 33. Thethermally switched optical filter of claim 32 further comprising adiffuser or de-concentrating lens that disperses an intensity of theemitted light from the thermally switched optical filter.
 34. Thethermally switched optical filter of claim 1 further comprising a colorfilter overlaying one or both of an inner surface or an outer surface ofthe thermally switched optical filter.
 35. The thermally switchedoptical filter of claim 1 further comprising an antireflective coatingoverlaying one or both of an inner surface or an outer surface of thethermally switched optical filter.
 36. The thermally switched opticalfilter of claim 1 further comprising a thermal insulation layersupported by the substrate.
 37. The thermally switched optical filter ofclaim 36, wherein the thermal insulation layer further comprises an airgap.
 38. The thermally switched optical filter of claim 1 furthercomprising a broadband reflector that reflects the emitted lightinternally within the thermally switched optical filter.
 39. Thethermally switched optical filter of claim 1 further comprising anenergy storage material supported by the substrate.
 40. A thermallyswitched optical filter comprising a first bandblock filter layer; asecond bandblock filter layer; a thermochromic downconverter layersandwiched between the first bandblock filter layer and the secondbandblock filter layer, wherein the downconverter layer absorbs incidentlight of a broad bandwidth and emits light at an emission wavelengthsubstantially or entirely longer than wavelengths of the broadbandwidth, and the emission wavelength is variable depending upon atemperature of the downconverter layer; and a transparent substratesupporting the first bandblock filter layer, the second bandblock filterlayer, and the thermochromic downconverter layer; wherein the firstbandblock filter layer blocks the emitted light when the temperature ofthe downconverter layer is below a first threshold temperature andpasses the emitted light when the temperature of the downconverter layeris above a second threshold temperature; and the second bandblock filterlayer passes the emitted light when the temperature of the downconverterlayer is below the first threshold temperature and blocks the emittedlight when the temperature of the downconverter layer is above thesecond threshold temperature.
 41. The thermally switched optical filterof claim 40, wherein the emission wavelength of the downconverter occursin the visible spectrum.
 42. The thermally switched optical filter ofclaim 40, wherein the emission wavelength of the downconverter occurs inthe infrared spectrum.
 43. A spandrel comprising the thermally switchedoptical filter of claim
 40. 44. A window comprising the thermallyswitched optical filter of claim 40.